The invention relates to the layer composition of an electrowetting system with a first electrode layer, an insulator layer on the first electrode layer, and a fluid layer above the insulator layer, with the fluid layer comprising at least two immiscible fluids which, under the influence of an applied voltage, reversibly change their wetting behavior of a surface allocated to the insulator layer.
Such a layer composition is described in the article “Electrowetting Displays” by Johan Feenstra and Rob Hayes, accessible under
http://www.liquavista.com/files/LQV060828XYR-15.pdf.
Electrowetting describes a procedure to modify the form of a fluid, or to change its position, by applying an electrical field. The form of a drop is determined by its surface tension relative to the adjacent media. The surface tension is an effect of the associated surface energies, which, in the case of electrowetting, are influenced by an electrostatic contribution. This relationship is described by the Young-Lippmann equationγLV cos θ=γSV−γSL+(½)(∈0∈r/d)V2 with γLV, γSV and γSL as well as ∈r being material constants, so that angle θ that defines the tangent of a drop at the interface with a solid base is changed when voltage V is changed, and ∈r being regarded as predefined by the fluid(s).
In the first electrode layer, various variants of electrode configurations may be arranged, depending on whether only the contour of the drop should be modified, whether the position of the drop should be changed through structured electrodes, or whether a combination of both effects is desired.
In the known layer compositions, materials with good insulating properties and high dielectric strength are used as insulating layer. The above mentioned article proposes glass, i.e. SiO2.
The invention is based on the finding that not only the materials of the fluid, but also other portions contribute to the Young-Lippman equation.
The voltage applied to the electrowetting system distributes differently onto the various areas. The overall behavior can be described as a surrogate circuit diagram with a serial connection of capacitances. The voltage partly drops at the fluid drop, partly at the wetted interface with the insulator and, if provided, also at a cover electrode. However, due to the high permittivity of the used fluids the voltage drop over the drop height is comparatively small. Thus, in known electrowetting systems, the field is applied mainly to the insulator layer over the electrodes.
For a satisfactory electrowetting effect, field strengths of approximately 70% of the dielectric strength are required. Very small irregularities in the thickness then result in the drop, inducing a field breakdown with corresponding destruction when it reaches such a site. A thicker insulator layer, as may be usually used in such cases, is not helpful in this case, as by increasing the layer thickness the field portion in the area of the drop decreases significantly. This again makes it necessary to increase the voltage, which again approximates the field at the insulator layer to the dielectric strength.